A prior art abrasive sheet is shown in FIG. 3 and is prepared by mixing and stirring abrasive particles, adhesive as a binder, e.g. a polyester resin and so on, and a hardener, uniformly painting and dry-hardening the mixture on a plastic base sheet, e.g. a polyester sheet and so on. 1 is the base sheet, 2 is the abrasive layer, 3 and 3a are abrasive particles and 4 is the binder.
In such an abrasive sheet which is manufactured by the prior art process, part 3a of the abrasive particles 3 which emerge from a surface of the abrasive layer 2 contribute to polishing, however, another of the abrasive particles are buried in the abrasive layer and never contribute to polishing.
Accordingly, some abrasive preparing processes to form a thin abrasive layer on a base sheet so that abrasive particles buried in the abrasive layer decrease are developed. However, for instance, when a thin abrasive layer is thinly formed on a base sheet so that abrasive particles are adhered on the base sheet in monolayer, as shown in FIG. 4, the flatness of the abrasive sheet is no good because of differences 5 among diameters of abrasive particles.
In another process for preparing an abrasive sheet using a roller (FIG. 5a) or a blade (FIG. 5b) in order to improve flatness of the abrasive sheet, larger abrasive particles 3'catch on the roller 6 or the blade 7, move together and damage a surface of the abrasive layer 4.
Because there are a way of densely heaping up abrasive particles on a sheet (FIG. 6a) and a way of simply heaping up abrasive particles on a sheet (FIG. 6bl) in forming a abrasive layer on a base sheet, as shown in FIG. 6, even if abrasive particles which are ideally all of the same size are used, a abrasive sheet with good flatness cannot be formed unless a abrasive layer is formed on a base sheet so that the thickness of the layer is several times a diameter of a abrasive particle, as shown in FIG. 3. Thus, in such a abrasive sheet, only part of the abrasive particles which emerge from a surface of the sheet only make contribution to polishing, another of the abrasive particles which are buried in the abrasive layer never make contribution to polishing. This causes increase in preparing cost, when expensive abrasive particles such diamond and so on are used as abrasive particles.
The present invention is directed to overcoming the shortcomings of the prior art as set forth above, therefore, an object of the present invention is to provide a process for preparing an abrasive sheet with a thin abrasive layer over which expensive abrasive particles such diamond are sparsely spread, and having good flatness.